Ethylene is a colorless flammable gas with a formula of C2H4. Ethylene is a basic chemical that is used widely for production of ethylene derivative chemicals. Major industrial reactions using ethylene include polymerization, oxidation, halogenation and hydro halogenation, alkylation, hydration, oligomerization and hydroformylation.
Ethylene is produced in the petrochemical industries from various types of feedstocks such as ethane, propane, ethane-propane mix, butane, naphtha, etc. through the process of steam cracking or in the oil refineries by cracking over zeolite catalysts. Typical process design in the production of ethylene includes feed treating, steam cracking, heat recovery, acid gas treatment, cracked gas compression, cold fractionation and hot fractionation.
In the cold fractionation process, due to the extremely cold temperatures, aluminum heat exchangers are usually used because of the compatibility of their metallurgy with various other interconnected parts of the system. In addition, aluminum heat exchangers are effective in lowering overall equipment count and capital investment required for an ethylene manufacturing plant.
In general, the aluminum heat exchangers are used to transfer heat between multiple streams in a “cold box.” Heat exchangers can be used alone or in combination in the same cold box. Usually, the “hot streams” connected to the aluminum heat exchangers come from various levels of refrigeration and transfer heat to “cold streams” from cold service process equipment.
While operating, temperatures across an aluminum heat exchanger can range from −350 F to 160 F, depending on the plant design. Such a great temperature range induces tremendous thermal stresses. The thermal stresses often lead to premature aging of the aluminum heat exchanger and fracture failure.
In order to overcome the problem of failure, the prior art has responded by reinforcing the mechanical design of the aluminum heat exchangers. However, despite design improvements, the process piping and equipment connecting to the outlet streams of the aluminum heat exchangers are often made of interior materials, such as ductile iron. Inferior materials are not a problem so long as the system operates in normal temperature ranges. However, the inferior materials can lead to catastrophic failure when exposed to extremely low temperatures, which sometimes result from abnormal plant operating conditions. Such catastrophic failure results in plant shut down and can result in injury to plant personnel. Superior materials for process piping, such as stainless steel, are available which can operate safely across larger temperature ranges, such as below −20 F, but they are extremely expensive and difficult to fabricate.
Examples of manufacturing processes which involve low temperatures are found in the prior art but they do not solve the problems inherent in abnormally low process temperatures. U.S. Pat. No. 5,361,589 to Howard, et al. discloses an ethylene recovery system with cracked gas cooled to about −20 to −40 F. However, Howard does not disclose how to control temperatures to protect process piping and equipment during abnormal operating conditions where lower temperatures are experienced.
U.S. Pat. No. 5,979,177 to Summer, et al. discloses an ethylene plant refrigeration system where the gas feed is cooled to about −31 to −35 F. However, Summer does not disclose how to monitor stream temperatures and control temperatures to protect process piping and equipment during abnormal low temperature conditions.
U.S. Pat. No. 4,900,347 to McCue, et al. discloses a method for recovering ethane or ethylene from cracking gas requiring low temperature refrigeration. At least one portion of the disclosed method has process temperatures below −20 F. However, McCue does not disclose how temperatures of process streams are monitored or modified in response to abnormal low temperature conditions.
Other prior art demonstrates methods to monitor and control process temperatures. However, none of the methods have been entirely satisfactory in controlling abnormal low process temperatures while protecting process piping and equipment.
U.S. Pat. No. 4,488,239 to Agarwal discloses a system to control temperatures in an olefin oxidation reactor by incrementally adjusting the flow rate of coolant to the chemical reactor based on measured temperatures. However, Agarwal does not disclose control of streams based on abnormally low process temperatures around the reactor to protect process lines and equipment.
United States Patent Publication No. 2010-026301 filed by Schwartz, et al. discloses a method for controlling a process flow rate through an aluminum heat exchanger by adjusting a bypass. However, Schwartz does not disclose the use of redirection or flow stoppages.